The term zeolites is generally understood to mean alkali metal or alkaline earth aluminosilicates of the general formula EQU M.sub.2/n O . Al.sub.2 O.sub.3 . y SiO.sub.2 . z H.sub.2 O
wherein M is a univalent or multivalent metal, H, NH.sub.4, CH.sub.3 -NH.sub.3, etc. and n indicates its valence, y lies between 1.8 and about 12 and z lies between 0 and 9. Their crystal lattice consists of cubo-octahedra with corner points of SiO.sub.2 and AlO.sub.4 tetrahedra. Characteristically they form a system of identically structured cavities with pore openings of the same size therebetween.
Among other purposes, zeolites are used for selective adsorption as well as for catalytic processes. In petrochemistry they are used, for example, for separation of isomeric hydrocarbons and also as catalysts in hydrogenation, isomerization and cracking of hydrocarbons. Synthetic zeolites of the faujasite type are particularly important for this application. Mainly zeolite X with y=2 to 3 and zeolite Y with y=3 to about 50 are used as catalysts.
The SiO.sub.2 /Al.sub.2 O.sub.3 ratio is essentially fixed during the synthesis of the zeolites. A subsequent change thereof is possible to only a very limited extent if other properties, especially the crystallinity, are not to suffer thereby. Nevertheless, for many fields of use, the desire exists to change the ratio as widely as possible, since positive effects are associated therewith; thus zeolites richer in silica prove to be more resistant to high temperature and acids. Moreover, they influence the selectivity when they are used in catalysts.
Intensive work has therefore been done on increasing the silicon-to-aluminum molar ratio.
Thus the following aluminum-removal processes are known: steam treatment at high temperature of, e.g., 700.degree. C. (see U.S. Pat. No. 3,591,488), calcining of NH.sub.4 Y zeolites, extraction with chelating agents such as EDTA, acetylacetonate, etc. (see West German Laid-open Applications 1,467,149 and 2,061,285), leaching with acids and bases (see East German Patent 121,331) and reactions with halogen or acid in the gas phase, e.g., F.sub.2, Cl.sub.2, HCl, etc.
Attempts have also been made to replace the aluminum by silicon. Silicon compounds such as SiCl.sub.4, SiF.sub.4, (NH.sub.4) .sub.2 SiF.sub.6, etc. have been used for this purpose.
In "Catalysis by Zeolites", 1980, Elsevier, Amsterdam, page 203 ff., Hermann K. Beyer and Ita Belenykaja describe the synthesis of a highly crystalline zeolite of the faujasite type, essentially indicating first a reaction with gaseous SiCl.sub.4 between 730 and 830.degree. K. and then a washing treatment with water. The reaction obviously does not begin until 730.degree. K.
In European Patent A2-72,397, a process was described for synthesis of ultrastable zeolites of the Y type by thermal removal of aluminum from activated zeolites of the Y type using gaseous halosilanes under conditions of exclusion of water; the activated zeolites have to be reacted at temperatures of 150.degree. to 450.degree. C., preferably 200.degree. to 400.degree. C., especially between 250 and 380.degree. C. The reaction product is washed with water until free of halogen. In the preferred temperature ranges, zeolites with cell constants a.sub.o of 2.442 to 2.423 nm and SiO.sub.2 /Al.sub.2 O.sub.3 ratios of 7 to 39 are obtained. No mention is made of the crystallinity.
In European Patent B1-82,211, a process is described for introduction of silicon atoms into the crystal lattice of an aluminum zeolite at temperatures between 20.degree. and 95.degree. C. by reaction with a fluorosilicate salt. Disadvantages therein are that the sodium ions must be exchanged by ammonium ions before aluminum removal, that expensive monitoring provisions are necessary because of the pH constancy and that the handling of fluoride compounds is problematic because of corrosion, toxicity and environmental protection.
In European Patent Al-62,123, a process is described for aluminum removal from aluminosilicates with inorganic halides, such as, e.g., SiCl.sub.4, at elevated temperature sufficient for volatilization of the formed Al compounds. When the halogen is Cl, the temperature lies preferably betwen 140.degree. and 760.degree. C. In the examples, processing takes place at 540.degree. C. In such a case the removal of the reaction products lasts 1 to 7 days.
In U.S. Pat. No. 3,644,220, a process is described for treatment of crystalline zeolites while largely preserving their crystallinity, the zeolites being allowed to react with volatile halogen compounds, including those of silicon, at elevated temperature, especially at 200.degree. to 650.degree. C. (400.degree. to 1200.degree. F.) and then being washed with water or aqueous ammonium hydroxide.